United Kingdom Patent No. 2398648 describes a particularly advantageous modulated supply stage comprising a switched supply stage and an error correction stage. The switched supply stage selects one of a plurality of supply voltages in dependence on a reference signal representing, in the preferred implementation, a signal to be amplified by an RF (radio frequency) amplifier. The error correction stage comprises an error correction amplifier and provides an error correction voltage for a correction of an error in a switched supply voltage, to deliver a more accurate supply voltage to the RF amplifier.
The error correction stage is provided to enable fast correction of the switched supply stage. There is a trade-off between output power and bandwidth. An increase in output power tends to result in a reduction in bandwidth due to increased parasitic elements of the higher power correction amplifier and combining components.
Further, when a transformer is advantageously used to combine the switched supply voltage and the error correction voltage, the increase in transformer size necessary for increased power handling results in increased leakage inductance and inter-winding capacitance, and a consequent loss of bandwidth.
It is an aim of the invention to provide an improvement to the advantageous modulated power supply described in United Kingdom Patent No. 2398648, and particularly to provide improvements in the error correction stage.